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To establish reliable trends, several more years of data are needed, said University of Colorado Denver Professor Callie Rennison, an expert in crime reporting.

“This is really not much time to know anything,” she said. “Give me five years worth of data or three or four data points and we can tell if reporting is up.”

Nevertheless, House Administration Chairman Dan Lungren said he has noticed ebbs and flows.

“At certain times, you take a certain snapshot and you see you have more and you go another six months and you have less,” the California Republican said.

For his part, Gainer said he can tell some things about the people who make threats. Many times, he said, alcohol or mental illness is a factor, though he did not give specifics.

This year, the contents of the threats are more varied, Gainer said.

“The broad range I’d classify as social economic concerns, health care, veterans’ assistance, Social Security, taxes,” he said. “There wasn’t one thing, unlike what we saw during the health care debate. During that time frame, everything was very much oriented toward the health care debate.”

The FBI closed 20 investigations into threats reported from October 2009 to March 2010, less than half of the total number of threats reported during that time period, according to FBI records obtained in early April through the Freedom of Information Act.

Of those, six threats specifically mentioned that the threat pertained to the health care debate. In only three of the cases did a court carry out punitive action.

Eight cases were closed after the investigation ran cold, while in another eight investigations, agents made contact with the perpetrator and a U.S. attorney declined to prosecute. The outcome of the last case was unclear.

As of early April, investigations into just four threats made from October 2010 to March 2011 have been closed.

Since the FBI only releases records of closed cases, it is unclear how many threats were investigated and how many remain unsolved.

A Closer Look at Six Closed Cases of Threats

The FBI has closed investigations into at least 26 cases of threats sent to Members of Congress in 2010. Here are a few representative cases:

When the FBI traced the call and interviewed the man, it turned out he was angry about receiving a robocall about health care.

“At the end of the call, the receiving party was told that they could be connected directly to their Senator if they pressed ‘1.’ The party was connected to Sen. Feingold’s office and the threatening message was left,” according to the FBI file. The man “advised that he was upset with the telemarketer and did not intend to threaten Sen. Feingold.”

The case was closed without prosecution.

• On Feb. 26, 2010, the day then-Sen. Jim Bunning tried to block legislation to extend unemployment benefits on the Senate floor, a man called the Republican’s Louisville, Ky., office.